Field of the Invention
The present invention relates to an image forming apparatus changing a circumferential speed ratio of a developer bearing member to an image bearing member to reduce a variation in image density of a long sheet.
Description of the Related Art
In POP (point of purchase) advertisement as an advertising medium for sales promotion mainly used in shops, due to a high printing ratio, a solid image or a long sheet such as a recording material of which length is in a range of 900 mm to 1200 mm is used. If a solid image is printed on such a long sheet, there occurs a problem in that an image density is lowered in a latter half of the image. This problem is a phenomenon occurring when a consumed amount of the toner on the surface of the developing roller is large and, thus, a supplied amount of the toner to the developing roller is insufficient.
As measures to the problem, the inventor of the present invention found that an image density stability of the long sheet can be secured by lowering the developing efficiency by an electric field and increasing a circumferential speed ratio of the developing roller to a photosensitive drum. Herein, configurations of changing the circumferential speed ratio of the developing roller to the photosensitive drum are disclosed in the following documents.
Japanese Patent Laid-Open No. 2-245778 discloses a configuration of changing modes by using an electronic clutch so that in a period of a character (standard) mode, a hard contrast image is formed by setting a rotation speed of the developing roller to be high, and in a period of a picture mode, a soft contrast image is formed by setting the rotation speed of the developing roller to be low. According to this configuration, a small-sized, low-cost image forming apparatus can be provided.
Japanese Patent Laid-Open No. 2005-189279 discloses a configuration of detecting the number of times of image formation and changing a rotation speed of a developing sleeve. According to this configuration, a stabilized image can be provided irrespective of the number of times of image formation.
As the other related arts, in a developing system for a liquid developer, a configuration of changing the rotation number of a developing roller based on a type of a recording material is disclosed. According to this configuration, a uniform developer density can be obtained.
In addition, as the other related arts, a configuration of changing a rotation speed of a developing roller based on a detection value of a toner density detection portion is disclosed. According to this configuration, an appropriate toner density is maintained.
However, in the inventions of Japanese Patent Laid-Open No. 2-245778 and No. 2005-189279, and the other related arts, there is no mention about a density stability of a solid image on a long sheet. Herein, a developing efficiency by an electric field is allowed to be lowered, and a consumed amount of toner on the surface of the developing roller is allowed to be decreased. By doing so, a residual toner amount on the surface of the developing roller is increased, and a circumferential speed ratio of the developing roller to the photosensitive drum is increased, so that a supplied amount of toner to the surface of the developing roller is increased. As a result, the problem in that the image density is lowered in the latter half of the image of the long sheet can be alleviated, so that a uniform image on the recording material can be obtained.
On the other hand, if the circumferential speed ratio of the developing roller to the photosensitive drum is increased, the abrasion of the photosensitive drum or the developing roller is increased, so that there occurs a problem in that the life cycle of the photosensitive drum or the developing roller is shortened.